Power output stage with limited current absorption during high-impedance phase

Abstract: A power stage of quasi-complementary symmetry, including a common-source FET and a common-drain FET, with a reduced absorption of current under the conditions of high impedance of the output. The driving node of the upper (common-drain) transistor is decoupled from the output node of the stage, preventing the current generator I d , which discharges the control node, from absorbing current from the load connected to the output stage, during a phase of high output impedance. This is preferably realized by using a field effect transistor which has its gate connected to the output node of the stage, and is connected to provide the current drawn from the discharge generator of the driving node of the upper common-drain transistor, absorbing it from the supply node VDD instead of absorbing it from the voltage overdriven node Vb. This alternative solution avoids excessive loading of the high-voltage supply, and is particularly useful when the overdriven node Vb drives multiple output stages.

Description: The present invention generally concerns circuits for controlling power FETs, and particularly a power stage of quasi-complementary symmetry, i.e . a stage which includes a first transistor in common-source configuration and a second transistor in common-drain configuration, for driving loads of mixed type. Half-bridge power stages are commonly used for driving brushless motors, stepper motors, and for transmitting logic signals at high-voltage. When the technology of fabrication permits, it is preferable to utilize field effect transistors for the output, for example, VDMOS (Vertical Double Diffused MOS transistors) or LDMOS (Lateral Double Diffused MOS transistors), since these provide significant advantages over bipolar transistors of comparable current-carrying capacity. In a typical half-bridge stage, the two transistors of the stage are connected in series between the two power supply nodes, and are driven alternatively into conduction by appropriate control circuits which are coupled to their gate terminals. While it is possible to use a complementary pair of output transistors (i.e. PMOS and NMOS), this solution is seldom used because a greater area is required for a PMOS transistor (as is well known, for a PMOS transistor the ratio between the internal on-resistance Ron and the area required is more than double the ratio for a comparable NMOS transistor). The necessity to minimize the area conversely favors the use of a pair of output transistors which are both N-channel, to realize a power stage which is of quasi-complementary symmetry including a first transistor (or stage) in common-source configuration, at an output node of which a second transistor (stage) in common-drain configuration is connected. As is well known, a common-drain stage (or transistor) requires a gate drive potential (Vb) which is higher than the common potential of the drain (VDD), in order to assure a suitable turn-on. Such an elevated drive voltage Vb is commonly generated within the integrated circuit, by means of a voltage-multiplying circuit. Moreover, the load driven by a half-bridge power stage will almost always be a reactive load, and often will have "mixed" characteristics. For example, in the case of electric motors, the load acts like a capacitive load at start-up, but while the velocity is regulated toward regime ( i.e. in phases where reactive current is being recirculated), the load behaves like an inductor. In the presence of voltage inertial loads (capacitive loads), it is necessary to protect the transistor from the inevitable voltage spikes which may cause the gate/source breakdown voltage to be exceeded (particularly in view of the overdriven gate of the common-drain-configured transistor). This protection function is commonly handled by at least one pair of back-to-back zener diodes, connected between the gate and source of the transistor which is overdriven by the high voltage Vb on its gate. This problem does not exist for the other power transistor, since the source of this other transistor is always held at a common potential (ground potential usually). The two figures 1A and 1B show a typical circuit configuration for a half-bridge power stage, which includes a pair of FETs M1 and M2. The upper transistor (M2) is overdriven by a gate voltage Vb which is higher than its drain voltage VDD. The drive of the transistor M2 is commanded by a pair of signals SWD and SWS, which are respectively connected to charge and to discharge the gate node of the transistor M2, by means of respective current generators Is and Id. Figure 1A shows the case of a capacitive load. In this operating regime, the zener diodes D1 and D2 limit the gate-source voltage V GS of the transistor M2, when it is turned on in the presence of voltage inertial loads (capacitive load CL). When the zener protection diodes D1 and D2 are turned on, they can carry a current Is, with forward and reverse bias respectively. In an inductive load condition, as shown in Figure 1B, the zener protection diodes D1 and D2 limit (in the opposite direction) the maximum gate-source voltage V GS of the transistor M2 (which in the illustrated phase is off), while the transistor M1 makes the transition from on-state to off-state. In these conditions, which may be seen for example during the speed-regulation of an electric motor, the output voltage Vo, due to the effect of the current Io which flows through the inductive, load (discharge current of the energy stored in the inductance Ll), and across the recirculation diode D3 intrinsic to the integrated structure of the FET M2, rise to a value given by: Vo = V DD +V D3 , which is a value higher than the supply voltage V DD . The zener diodes D1 and D2 therefore are turned on and carry, in forward and reverse bias current directions respectively, a current Id. Under these conditions, a current Ilo equal to Id is absorbed at the output node Vo. This current absorption across the output node of the power stage begins when the output voltage Vo exceeds a threshold value given by: Vo t =V D2 +V z , where V D2 is the drop across forward biased zener D2 and V z is the zener voltage of the diode D1. Therefore the threshold value Vo t is necessarily less than the maximum value of the output voltage Vo. This current IIo (due to the presence of zener diodes for protection of the common-drain-configured transistor flows), is absorbed under conditions of high output impedance, and introduces some imprecision in the current which flows through the load. Indeed, during the phases of recirculation (with M2 off), and when the voltage of the output node exceeds the threshold voltage (Vo>V D2 +V z ), the circuit absorbs a current Id from the load through the protection diodes D1 and D2. This current absorption changes the time constant for discharge of the inductance, and introduces therefore an imprecision which may be intolerable in many applications. Another drawback is represented by the fact that, during testing of the integrated circuits, the presence of the current absorbed at the'output node may obstruct verification of the high-voltage integrity of the common-source connected transistor (M1). Indeed, the absorption of current across the protective diodes D1 and D2 prevents determination of absorption dye to possible leakage current into the substrate of the transistor M1. It is therefore a primary object of the present invention to provide a power stage of quasi-complementary symmetry, including a common-source FET and a common-drain FET, with a reduced absorption of current under the conditions of high impedance of the output. This object is achieved by a power stage as specified in claim 1. The invention reduces the absorption of current by substantially decoupling the driving node of the upper (common-drain) transistor from the output node of the stage, preventing the current generator I d , which discharges the control node, from absorbing current from the load connected to the stage output, during a phase of high output impedance, and when the inertial swing of voltage on the output node exceeds the threshold value which causes turn on of the zener protection diodes connected between the output node of the stage and the control node of the upper (common-drain) transistor. An effective decoupling may be realized by using a bipolar transistor, having its base connected to the output node of the stage, and effective to provide the current drawn from the discharge generator of the driving node of the upper common-drain transistor, absorbing it on the high-voltage overdriven supply node. In practice, the current absorbed from the load turns out to be a fraction of the current from the discharge current generator, in a relation given by the current gain of the decoupling transistor. Therefore, the decoupling transistor may be realized by a transistor of high gain, or alternatively a Darlington stage. To absorb the current drawn by the discharge generator from the node at overdriven voltage seems to be a drawback, because this overdriven voltage is usually achieved charging a capacitor across a synchronous rectifier (charge pump), or across a voltage multiplier, in which also has limited capacity for current draw. To this end, it may also be considered that during the absorption, the overdriven transistor is off, and hence momentarily does not require overdriving from the high voltage. According to an alternative embodiment of the invention, even this possible disadvantageous aspect is superseded by providing circuitry for absorbing the necessary current from the supply node VDD instead of absorbing it from the voltage overdriven node Vb. This alternative solution is particularly useful when the overdriven node Vb drives multiple output stages. According to this alternative embodiment of the invention, the decoupling is obtained by using a field effect transistor instead of a bipolar transistor. This permits absorption of the current directly from the supply line VDD in order to not overload the overdriven node Vb. By providing a FET instead of a bipolar transistor, the problem of saturation of the bipolar transistor, when connected to the supply line VDD, is avoided. Moreover, when a condition of saturation is seen, the drastical reduction of the bipolar transistor current gain (causing a losing of the capacity to effectively decouple the output node of the stage) is also avoided. Naturally, this problem, related to saturation, is not present when a FET transistor is used for decoupling, since saturation for an FET follows a purely resistive type of behavior of the transistor. The different aspects and advantages of the invention will be more evident through the following description with reference to the accompanying drawings, which show important sample embodiments of the invention, wherein: Figures 1A and 1B are simplified diagrams of a power stage. In particular, of a system for driving and for protection from overvoltages of the upper transistor in the common-drain configuration respectively in the case of a capacitive load and in the case of an inductive load, as described above. Figure 2 shows a diagram of a power stage modified according to the present invention. Figure 2bis shows an alternative embodiment of the invention, including a pair of transistors in Darlington configuration for decoupling. Figure 2tris shows, a further alternative embodiment of the invention, which is effective to avoid loading of the high voltage driving node. Figures 3a and 3b are respectively plan and section views of an integrated circuit structure of an embodiment of the invention according to Figure 2. With reference to Figure 2, the additional bipolar transistor Q1 fills the function of decoupling the output node of the power stage from the generator Id when the output is into a high impedance condition (with M1 and M2 off). In this condition, when the voltage on the output node exceeds the limit value ( V o >V DD +V D3 ) the zeners D1 and D2 turn on and receive the current Id, driven from the discharge generator of the gate node of the transistor M2. This current is supplied from the decoupling transistor Q1, which absorbs it from the high voltage node Vb. Meanwhile the absorption of current from the load (according to the scheme described in relation to Figure 1b) will therefore be given by: I lo =I d /β Q1 . Clearly this absorption can be reduced as much as necessary, by providing a decoupling transistor Q1 having an adequate current gain. Alternatively, as shown in Figure 2bis, a Darlington stage can be used, formed by a cascaded pair of transistors Q1a and Q1b, in which case the current therefore absorbed from the load is therefore given by I lo =I d /(β Q1 ) 2 . In both these embodiments (of Figure 2 or Figure 2bis), the base-emitter junction of the decoupling transistor Q1 (or of the decoupling transistors Q1a and Q1b), will be protected when when reverse biasing occurs, by the presence of the zener diode D2 (or of D2a and D2b in the latter case). According to the embodiment of Figure 2bis, using a Darlington stage, the two diodes D2a and D2b should each have a zener voltage equal to half the zener voltage necessary to effectively protect the gate-source junction of transistor M2. Naturally, diodes D1a and D1b may be alternatively replaced by a single zener diode having a zener voltage equal to the sum of the zener voltages of D1a and D1b (or of D2a and D2b). As noted above, current consumption from the high voltage node V b may be avoided, in cases where this might be a problem, by using the alternative form of the invention shown in Figure 2tris. According to this alternative embodiment, an FET M3 is used for decoupling the output node of the stage from the generator I d , when, under conditions of high impedance of the output node (M1 and M2 off), the voltage on the output node exceeds a threshold voltage which in this case will be given by: V gsM3 < V z + V be at current value I d provided from the discharge current generator of the node for driving the output transistor M2. This condition may easily be established by appropriately selecting the channel size of transistor M3. A diode D4 is added in series with the decoupling transistor M3 to prevent the intrinsic diode D5 (which is intrinsic to the structure of decoupling transistor M3) from turning on, while transistor M2 turns on, since this would short circuit the high voltage line V b to the supply voltage line V DD . This alternative solution avoids overloading the high voltage line V b , and allows absorbtion of the current drawn from transistor I d directly from the supply node V DD . In integrated circuits, the invention is particularly useful when zener diodes with a breakdown voltage V z less than V eb0 (breakdown voltage of the emitter-base junction with collector open) of the bipolar decoupling transistor Q1 are available. Moreover, in all the situations in which such zener diodes are not available, the alternative solutions described in relation to Figures 2bis and 2tris will overcome possible difficulties of implementation, because (in the case of the solution of Figure 2bis) the condition V z < V eb0 is always satisfied. In the case of the solution of Figure 2tris, obviously there is no need to protect any reverse biased junctions. In this case, the functionality of the arrangement is assured by the appropriate dimensioning of the MOS transistor M3 so as to preserve the relation Vz <V gsD3 , to avoid an improper turn-on. A simple illustrative implementation of the invention is in a half-bridge power stage, comprising two N-channel LDMOS transistors having W/L dimensions (in microns) between 20,000/4 and 40,000/4, for use with specified supply voltages VDD in the range between 10 and 18 volts. In this example, the upper transistor, connected in common-drain configuration, is overdriven with a high voltage Vb of about 25 volts. In such an output stage, the generator for discharging the driving node of the upper transistor may be configured to supply a discharge current I d of about 1 milliAmp. According to the present invention, the NPN decoupling transistor Q1 (Figure 2) or the two NPN transistors cascaded to form a Darlington pair (Figure 2bis), may have relatively small sizes, substantially equivalent to the minimum dimension of the process. Even if a decoupling FET (M3) is provided, in order to not load the high voltage node (Figure 2tris), the transistor may have relatively small dimensions. In the case of an output stage with the characteristics indicated above, the decoupling FET may have dimensions of, for example, between 20/4 and 50/4 in microns. It may be seen that the circuit provided by the invention is extremely simple and very economical in terms of area requirements. In the case of a solution as illustrated in Figure 2, implemented in an integrated circuit, an efficient integrated structure for the NPN decoupling transistor and for the zener protection diodes D2 is schematically illustrated in Figures 3a and 3b. The integrated structure is substantially that of a bipolar transistor of type NPN, in which the base emitter junction is protected by use of a deep P+ implant which allows the formation, with the N+ zone of the emitter, of a zener structure known as the "buried zener". The collector terminal of the NPN transistor is represented by an epitaxial layer N epi . For example, the P-body region may have a resistivity of 1000 ohms per square, the P+ regions may have a sheath resistance of 100 ohms per square, the N+ regions may have a resistivity of 40 ohms per square, and the epitaxial layer N epi may have a sheath resistance of 2100 ohms per square. Naturally, a composite structure of this type can be modified in various ways, by reflection, rotation, and translation of the active areas of the components (Q1 and D2). A further advantage obtained by the invention is the great simplicity of implementation of the drive circuit of the common-drain transistor M2. In particular, the discharge current generator I d may be designed with elevated peak values, in order to assure a rapid turn-off of the transistor M2, without undesirably increasing the current absorbed from the output node, due to the decoupling obtained by the arrangement of the invention.

Claims: A power stage comprising a first field effect transistor (M1) in common-source configuration, a second field effect transistor (M2) in common-drain configuration, said first and second field effect transistors being connected to an output node of the power stage, and driven alternatively into conduction by first and second control circuits (SWD, SWS, Id, Is) coupled to their respective gate terminals, said second field effect transistor (M2), connected common-drain configuration, being driven with a driving voltage (Vb) higher than the common voltage (VDD) of the drain thereof and at least one pairs of back-to-back zener limiting diodes (D1, D2), connected between source and gate of said second field effect transistor (M2) to limit the gate-source voltage thereof, characterized by further comprising circuitry for limiting the current drawn from a load connected to said output node of the power stage when said first and second field effect transistors (M1, M2) are turned off and said pair of back-to-back zener limiting diodes (D1, D2) is turned on, comprising at least a third transistor (Q1), controlled by said output node of the power stage, configured and connected to supply current absorbed by a current generator (Id) turning off said second transistor (M2) by drawing current from the gate terminal of said second field effect transistor (M2). The power stage according to claim 1, wherein said third transistor (Q1) is a bipolar NPN transistor, having a base connected to said output node of the power stage, an emitter connected to the intermediate connection node between said pair of zener limiting diodes (D1, D2), and a collector coupled to said higher driving voltage (Vb). The power stage according to claim 1, wherein said circuitry for limiting the current drawn from the load comprises first and second NPN bipolar transistors (Q1A, Q1B) connected in a Darlington configuration, having respective collectors thereof coupled in common to said higher driving voltage (Vb), said first NPN bipolar transistor (Q1B) being controlled by said output node of the power stage, and wherein at least the one (D2) of said back-to-back zener limiting diodes of said pair which is connected to said output node of the power stage is replaced by first and second zener diodes (D2A, D2B) connected in series, having each a zener voltage which is equivalent to half the required protection voltage for the gate-source junction of said first field effect transistor (M1) and being functionally connected between the emitter and the base of said first and second NPN bipolar transistor, respectively. The power stage according to claim 1, wherein said third transistor is a field effect transistor (M3) having a source connected to said gate terminal of said second field effect transistor (M2), and a drain connected through a directly biased diode (D4) to said common drain potential of said second field effect transistor (M2). The power stage according to claim 2, wherein said third bipolar NPN transistor (Q1) and the one (D2) of said back-to-back zener limiting diodes of said pair which is connected to said output node of the power stage are realized in the form of a hybrid integrated structure in which said zener diode (D2) is realized a a buried zener within a body region (Pbody) containing an emitter diffusion (N+) and a base diffusion (P+) of an epitaxial layer (Nepi) constituting a collector region of said third bipolar NPN transistor.

Inventors: BERTOLINI LUCA, BERTOLINI, LUCA
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: H02H9
IPC codes: H03K17, H02H9, H01L21, H01L29, H01L27